The invention is directed to a method of and an apparatus for control of safety valves in which a closing member is pressed against a valve seat against pressure of a medium under an action of an operating cylinder connected to a hydraulic control circuit, and in which, in order to achieve a valve opening lift, one or more outflow pressure sensor controlled solenoid valves open to enable a hydraulic fluid flow from the operating cylinder when the set pressure is reached. The solenoid valves close again when the pressure falls below the set pressure, whereby hydraulic fluid is supplied to the operating cylinder to effect valve closing.
In a known control method of this kind for safety valves, the operating cylinder constantly communicates with the pressure line of the hydraulic control circuit via a throttle. When the set pressure is reached, pressure switches open the outflow solenoid valves and keep them open until the medium pressure falls below a switching threshold of the pressure switches. Because of the continuously available load pressure, hydraulic fluid flows into the operating cylinder after the closing of the solenoid valves and closes the safety valve again.
A disadvantage in this known control device is that solenoid valves which are arranged in a redundant manner must have a valve opening area adapted to the highest pressure rise speed of the medium. Therefore, when the pressure rise is very slow, an excessive quantity of the medium to be secured is guided out and excessive pressure drop in the medium and high medium losses can accordingly occur. These pressure surges run through the pipeline as a pressure wave and lead to shaking of the safety valve and the pipeline. The pressure waves can also reach the pressure switches, so that the safety valve then continuously opens and closes in an undesired manner. In this known control device, the opening and closing of the safety valve is effected in accordance with an opening speed determined by the hydraulic system, and it is not possible in this case to adapt to an another opening and closing characteristic. Since the hydraulic pressure is continuously available at the operating cylinder via the throttle, the safety valve is closed again after every closure of the outflow solenoid valves, and permanent intermediate positions of the valve closing member are not possible. Therefore, this safety valve must continuously open and close when partial quantities are guided out.